1. Field of the Invention
This invention relates to a method and apparatus for capping cellular glass blocks utilized as a load bearing insulation for the base insulation of liquefied gas storage tanks and more particularly to the capping of the cellular glass block insulation with a layer of particulate vermiculite to provide a capping which results in improved compressive strength.
2. Description of the Prior Art
In the storage of liquefied gas such as liquefied natural gas (LNG) and liquid oxygen (LOX) in cryogenic strage tanks, cellular glass blocks are commonly utilized as a thermal insulation and a load bearing insulation for the tank base. To increase the compressive strength of the tank base, it is the practice to cap each course of the cellular glass block base insulation with a layer of capping material. This capping layer serves to increase the compressive strength of the cellular glass blocks. The capping material, however, should provide less than a 1/4 inch deformation of the total insulation system so that the tank base is not subjected to any major deformation when the tank is initially loaded. Also the capping should not be resilient so that the tank base is not elastically cycled when the tank is filled and emptied.
In the past, liquefied gas storage tank bases have been constructed with a layer of compressible material between the slabs of insulation. In the case of liquefied natural gas tanks, the capping layer is usually hot asphalt, an asphaltic based material or contains asphalt such as asphalt filled paper. In the base insulation of liquefied oxygen tanks, however, organic materials are not permissible, and asbestos paper is commonly used in place of asphaltic materials as a capping for the tank base insulation. Asbestos paper, however, as a capping layer presents a potential health hazard and is becoming less available. In addition to many conventional capping materials becoming commercially unfeasible because of high cost or unavailability, they exhibit properties which do not permit their use over a wide temperature range. Their properties are temperature sensitive, becoming brittle at cryogenic temperatures and fluid at elevated temperatures. Consequently, the use of such materials is restricted to the base insulation of tanks for the storage of liquids at cryogenic temperatures or at elevated temperatures, but not both.
Many of the conventional capping materials when applied as load bearing base insulation upon the surface of the cellular glass blocks tend to rebound from the surface of the blocks when compressed. The cellular glass blocks have a cut face that forms a plurality of open hemispherical cells. Preferably, the capping material should fill the cells to provide an effective capping layer between the courses of the cellular glass block base. Conventional capping materials having resilient properties may not effectively fill the open cells and subsequently the tank base is elastically deformed when the tank is filled and emptied. In addition, because these materials do not come into intimate contact with the cellular glass block surface, the compressive strength of such an insulation system is substantially reduced. For this reason they are ineffective as load bearing insulation.
There is need for an inorganic material for capping base insulation of liquefied gas storage tanks that completely fills the open hemispherical cells of the cellular glass blocks and comes into intimate contact therewith to form a capping layer that is irreversibly compressible to provide improved compressive strength for the base insulation. Furthermore, the inorganic capping material should be an effective loading bearing insulation at cryogenic temperatures and elevated temperatures as well.